A cable joint comprising at least one PILC type cable is typically used in the transmission of electric current at so-called “medium” voltages in the approximate range of 10 KV to 42 KV. In this type of cable, an inner conducting core made, for example, of twisted strands of a conducting metal is surrounded along a length of the cable by a cylindrical layer of paper that is impregnated with oil. The layer is in turn surrounded along the length of the cable by a lead (or other conducting material) jacket or sleeve. The impregnated paper layer acts as a dielectric that insulates the inner conductor from the outer (conductive) jacket. PILC type cables are rarely manufactured nowadays, but many tens of thousands of meters of the cable type remain in service around the world. Therefore, there remains a need for connecting the PILC type cables together, and for connecting the PILC type cables to other types of cable.
One characteristic of the PILC type cable is that if the oil migrates, the dielectric effect of the impregnated paper diminishes dramatically as the paper dries out. Generally this migration effect does not occur over the length of cable that lies away from the cable ends, but when it is required to splice an end of the PILC type cable, a potential problem arises in that leakage paths for the oil are created. Prior art designs of cable joint for use with the PILC type cables have sought to close off such leakage paths. This aim may be realized through the use of a rigid casing, of the kind described in GB 1 485 613, that encloses a joint assembly including various seals. The objective of including the seals is to prevent deleterious migration of the oil from the paper layer.
The arrangement of the joint assembly taught in GB 1 485 613, however, is complicated. Aside from the fact that he cable joint is expensive to manufacture, assembly of the cable joint is a lengthy process of steps that must be completed in the correct order in order to assure leak-proofing of the cable joint. If the cable joint is assembled in a “field” situation by an inexperienced fitter, there is a danger of the steps not being completed correctly or in the correct order, such that the cable joint fails in service. Similarly, if even one of the many components of the joint assembly taught in GB 1 485 613 becomes lost or damaged, the integrity of the cable joint is compromised.
A further problem with the joint assembly taught in GB 1 485 613 is that it does not seek to address the problems that can arise when the heating effect of electrical resistance in the cable of the PILC type causes expansion of the oil of the impregnated paper. Under such circumstances, the pressure of the oil within the joint can rise sufficiently such that the oil is forced to leak away, thereby reducing the dielectric effect. High oil pressures can also arise when the cable containing the cable joint lies, for example, on a hillside. The hydraulic head of the oil above the cable joint can then be adequate to promote the above-described migration of oil.
In another example, U.S. Pat. No. 5,374,784 teaches an arrangement in which the cable joint between two conducting elements of cables that are spliced together are encircled by an elastomeric sleeve. A heat-recoverable sleeve, for example, a sleeve made of a material that is pre-stressed to a shape used for fitting and that on heating reverts to a relaxed or “recovered” state, is used to surround the elastomeric sleeve. On heating the heat-recoverable sleeve shrinks to encircle the elastomeric member sufficiently tightly so that the oil cannot migrate into its interstices. Consequently, the oil can not enter into the material of the sleeve and a known effect of degradation of the sleeve does not, according to the disclosure of U.S. Pat. No. 5,374,784, occur.
The arrangement of U.S. Pat. No. 5,374,784, however, suffers from several disadvantages. First, the effect of the heat-recoverable sleeve is only of benefit while the pressure of the oil remains low. At higher pressures, the oil can force the elastomeric sleeve away from the cable joint and create voids. The oil can migrate into the voids and thereby cause failure of the insulation. Second, it is necessary for anyone fitting the cable joint of U.S. Pat. No. 5,374,784 in a “field” situation to carry a source of heat for heating the heat-recoverable sleeve. The need to carry such equipment has safety ramifications and generally complicates the process of creating a joint.
In a further example, JP 2000236619 discloses an arrangement in which an aluminum tape is used in conjunction with a complex structure in order to block oil leakage paths in the cable joint. A defect of this arrangement is that it is almost entirely rigid. Hence, this arrangement does not allow for small movements of the components of the cable joint without the integrity of the cable joint becoming compromised, which can lead to failure of the dielectric layer in service.